I think that Canidate could well end up as a good player in the right situation, but on this team there are too many running backs that can do what he does best - run fast.
Sellers is a tight end. We need a short yardage specialist - Rock you're the front runner there, but Betts could fill that role. Betts is a good all purpose back who can hopefully do well on special teams. McCullough I like as the primary backup for Portis, if he earns it in preseason. Yes, Morton is a good change of pace/3rd down back.
We may wait until preseason and see about trading one of our backs. If Johnson goes, we have:
Portis*
McCullough*
Morton*
Rock C. *
Betts*
Johnson (RFA to N.O. Saints)
Canidate (likely released)
John Simon (likely released)
Jimoh
*on roster - my projection